Mori Narumi, Nishiuma Kenta, Sugiyama Takuya, Hayashi Hideo, Akiyama Kohki
Graduate School of Life and Environmental Sciences, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan.
Phytochemistry. 2016 Oct;130:90-8. doi: 10.1016/j.phytochem.2016.05.012. Epub 2016 Jun 2.
Hyphal branching in the vicinity of host roots is a host recognition response of arbuscular mycorrhizal fungi. This morphological event is elicited by strigolactones. Strigolactones are carotenoid-derived terpenoids that are synthesized from carlactone and its oxidized derivatives. To test the possibility that carlactone and its oxidized derivatives might act as host-derived precolonization signals in arbuscular mycorrhizal symbiosis, carlactone, carlactonoic acid, and methyl carlactonoate as well as monohydroxycarlactones, 4-, 18-, and 19-hydroxycarlactones, were synthesized chemically and evaluated for hyphal branching-inducing activity in germinating spores of the arbuscular mycorrhizal fungus Gigaspora margarita. Hyphal branching activity was found to correlate with the degree of oxidation at C-19 methyl. Carlactone was only weakly active (100 ng/disc), whereas carlactonoic acid showed comparable activity to the natural canonical strigolactones such as strigol and sorgomol (100 pg/disc). Hydroxylation at either C-4 or C-18 did not significantly affect the activity. A series of carlactone analogues, named AD ester and AA'D diester, was synthesized by reacting formyl Meldrum's acid with benzyl, cyclohexylmethyl, and cyclogeranyl alcohols (the A-ring part), followed by coupling of the potassium enolates of the resulting formylacetic esters with the D-ring butenolide. AD ester analogues exhibited moderate activity (1 ng-100 pg/disc), while AA'D diester analogues having cyclohexylmethyl and cyclogeranyl groups were highly active on the AM fungus (10 pg/disc). These results indicate that the oxidation of methyl to carboxyl at C-19 in carlactone is a prerequisite but BC-ring formation is not essential to show hyphal branching activity comparable to that of canonical strigolactones.
在宿主根附近的菌丝分支是丛枝菌根真菌的一种宿主识别反应。这种形态学事件是由独脚金内酯引发的。独脚金内酯是类胡萝卜素衍生的萜类化合物，由独脚金醇内酯及其氧化衍生物合成。为了测试独脚金醇内酯及其氧化衍生物可能作为丛枝菌根共生中宿主来源的定殖前信号的可能性，化学合成了独脚金醇内酯、独脚金醇酸、独脚金醇酸甲酯以及单羟基独脚金醇内酯、4-羟基独脚金醇内酯、18-羟基独脚金醇内酯和19-羟基独脚金醇内酯，并评估了它们对丛枝菌根真菌珠状巨孢囊霉萌发孢子的菌丝分支诱导活性。发现菌丝分支活性与C-19甲基的氧化程度相关。独脚金醇内酯活性较弱（100 ng/圆片），而独脚金醇酸显示出与天然典型独脚金内酯如独脚金素和高粱内酯相当的活性（100 pg/圆片）。C-4或C-18位的羟基化对活性没有显著影响。通过使甲酰基丙二酸亚异丙酯与苄醇、环己基甲醇和环香叶醇（A环部分）反应，然后将所得甲酰基乙酸酯的烯醇钾盐与D环丁烯内酯偶联，合成了一系列名为AD酯和AA'D二酯的独脚金醇内酯类似物。AD酯类似物表现出中等活性（1 ng - 100 pg/圆片），而具有环己基甲基和环香叶基的AA'D二酯类似物对丛枝菌根真菌具有高活性（10 pg/圆片）。这些结果表明，独脚金醇内酯中C-19位甲基氧化为羧基是一个先决条件，但BC环的形成对于显示与典型独脚金内酯相当的菌丝分支活性并非必不可少。
